Bug 1478208 - Implement HTMLMediaElement.allowedToPlay. r=alwu,bz

Various web authors have expressed desire to know in advance whether autoplay
will work.

They want this in order to avoid paying the price for downloading media that
won't play. Or they want to take other action such as showing a poster image
instead.

This is of particular interest to Firefox, as we're planning on showing a
prompt to ask the user whether they would like a site to play. If sites want to
determine whether they can autoplay but avoid the prompt showing, they won't be
able to just call play() in Firefox and see whether it works, as that would
likely show the prompt if the user doesn't already have a stored permission.

We've been working out a spec here:
whatwg/html#3617 (comment)

This implements what is the consensus to date there;
HTMLMediaElement.allowedToPlay, which returns true when a play() call would not
be blocked with NotAllowedError by autoplay blocking policies.

MozReview-Commit-ID: AkBu0G7uCJ0

UltraBlame original commit: f5f55e363d318effdfd504061c50f87e06301c6f

dom/html/HTMLMediaElement.cpp

@@ -2522,6 +2522,12 @@ HTMLMediaElement::ResumeLoad(PreloadAction aAction)
   }
 }
 
+bool
+HTMLMediaElement::AllowedToPlay() const
+{
+  return AutoplayPolicy::IsAllowedToPlay(*this) == nsIAutoplay::ALLOWED;
+}
+
 void
 HTMLMediaElement::UpdatePreloadAction()
 {

dom/html/HTMLMediaElement.h

@@ -640,6 +640,12 @@ class HTMLMediaElement : public nsGenericHTMLElement,
     mIsCasting = aShow;
   }
 
+
+
+
+
+  bool AllowedToPlay() const;
+
   already_AddRefed<MediaSource> GetMozMediaSourceObject() const;
 
 

dom/media/test/AutoplayTestUtils.js

@@ -5,13 +5,13 @@ function playAndPostResult(muted, parent_window) {
   element.src = "short.mp4";
   element.id = "video";
   document.body.appendChild(element);
-
+  let allowedToPlay = element.allowedToPlay;
   element.play().then(
       () => {
-        parent_window.postMessage({played: true}, "*");
+        parent_window.postMessage({played: true, allowedToPlay}, "*");
       },
       () => {
-        parent_window.postMessage({played: false}, "*");
+        parent_window.postMessage({played: false, allowedToPlay}, "*");
       }
     );
 }

dom/media/test/test_autoplay_policy_activation.html

@@ -143,7 +143,8 @@
             await once(child, "load");
             child.postMessage(test_case, window.origin);
             let result = await nextWindowMessage();
-            SimpleTest.is(result.data.played, test_case.should_play, test_case.name);
+            SimpleTest.is(result.data.allowedToPlay, test_case.should_play, "allowed - " + test_case.name);
+            SimpleTest.is(result.data.played, test_case.should_play, "played - " + test_case.name);
             child.close();
           }
           SimpleTest.finish();

dom/media/test/test_autoplay_policy_permission.html

@@ -34,7 +34,8 @@
           child.postMessage("play-audible", testCase.origin);
           // Wait for the window to tell us whether it could play video.
           let result = await nextWindowMessage();
-          is(result.data.played, testCase.shouldPlay, testCase.message);
+          is(result.data.allowedToPlay, testCase.shouldPlay, "allowedToPlay - " + testCase.message);
+          is(result.data.played, testCase.shouldPlay, "played - " + testCase.message);
           child.close();
         }
 

dom/webidl/HTMLMediaElement.webidl

@@ -219,3 +219,13 @@ partial interface HTMLMediaElement {
   [Pref="media.test.video-suspend"]
   boolean hasSuspendTaint();
 };
+
+/*
+ * API that exposes whether a call to HTMLMediaElement.play() would be
+ * blocked by autoplay policies; whether the promise returned by play()
+ * would be rejected with NotAllowedError.
+ */
+partial interface HTMLMediaElement {
+  [Pref="media.allowed-to-play.enabled"]
+  readonly attribute boolean allowedToPlay;
+};

modules/libpref/init/all.js

@@ -594,6 +594,14 @@ pref("media.autoplay.enabled.user-gestures-needed", false);
 
 
 
+#ifdef NIGHTLY_BUILD
+pref("media.allowed-to-play.enabled", true);
+#else
+pref("media.allowed-to-play.enabled", false);
+#endif
+
+
+
 pref("media.video-queue.default-size", 10);
 
 

testing/profiles/common/user.js

@@ -46,3 +46,4 @@ user_pref("media.autoplay.default", 0);
 user_pref("media.autoplay.enabled.user-gestures-needed", true);
 user_pref("media.autoplay.ask-permission", false);
 user_pref("media.autoplay.block-webaudio", false);
+user_pref("media.allowed-to-play.enabled", true);